The object of the present invention is a consistency transmitter for the measurement of consistency, viscosity, and other such characteristics of matter. The object of the invention is particularly the consistency analyzers used in the pulp and paper industry.
In rotary consistency transmitters known in the prior art, a measuring element is rotated in the process being measured. Fibers and filler particles present in the measured matter, such as papermaking pulp, tend to resist the rotary motion of the measuring element. This resisting force, which is proportional to the shear force generated by the process matter, is measured using various torque measurement techniques and further converted into a variable indicating consistency. In prior art solutions the measuring element is rotated by a single-phase or three-phase motor that is located to the side of the rotation axis. The motor is connected to the torsion shaft by gear, chain, or belt drive transmission or some other power transmission system. Gearing is required in order to reduce the rotation speed. The use of single-phase or three-phase motor to rotate the measuring element poses several problems and limitations.
A three-phase motor suited for process conditions weighs from 6 to 10 kg, a single-phase motor over 10 kg. The motor has to be installed to the side of the torsion shaft, and thus the shaft forms a lever arm and the heavy motor causes a strong flexural and torsional stress at the point of the attachment to the process pipeline. Especially in process pipeworks, support structures must be installed, and these increase the investment cost. The bending and torsion caused by the weight of the motor also puts a strain on the torsion shaft. The weight of the motor and the required massiveness of the transmitter structure results in that the entire device may weigh over 30 kg. Thus, handling of the device requires several people or a hoist.
Power transmission elements require regular service and thus cause maintenance costs. For example, a transmission belt must be inspected for wear every six months and replaced every few years. The transmission belt causes extra bending strain on the torsion shaft and will thus contribute to more rapid wear of bearings and mechanical seals.
The rotation speed of single-phase and three-phase motors is dependent on the mains network frequency. The torque resisting the rotation of the measuring element increases exponentially as a function of both consistency and rotation speed of the measuring element. When the measuring element is rotated at a constant speed, its shape must be selected in accordance with the properties of the measured medium. The defining of suitable measuring elements causes additional costs and also increases the number of necessary spare parts.
Motor load, and thus also rotation speed, varies due to a number of reasons. Changes in rotation speed cause the measurement signal to drift and thereby complicate the measurement. Rotation speed changes when consistency, i.e. the shear force resisting measuring element rotation, changes. The rotation speed changes also due to changes in the friction of the torsion shaft bearings and mechanical seals. The friction forces in mechanical seals are affected by process pressure: a higher pressure forces the sealing surfaces more tightly against each other and thus increases friction. In cage induction motors, the backward slip that affects rotation speed is also dependent on motor load.
Most cage induction motors have a fixed direction of rotation, and as consequence, unwanted materials caught to the measuring element can only be removed if the device is first pulled out of the process.
Single-phase and three-phase motors require high-voltage operating power and must be well enclosed to protect them from moisture and the process environment. High voltage increases electrical safety requirements, and a qualified electrician is needed to install a three-phase motor to the power network or to disconnect it. Several single-phase and three-phase operating voltages are known in the world, and thus a separate motor type has to be chosen for each voltage. The need to provide for different operating voltages increases the variety of models and spare parts that a manufacturer has to offer.